24-89. Gray Code

题目描述：

The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer?n?representing the total number of bits in the code,print the sequence of gray code. A gray code sequence must begin with 0.

Example 1:

Input:?2
Output:?
Explanation:
00 - 0
01 - 1
11 - 3
10 - 2

For a given?n,a gray code sequence may not be uniquely defined.
For example,[0,2,3,1] is also a valid gray code sequence.

00 - 0
10 - 2
11 - 3
01 - 1
[0,1,2]

Example 2:

Input:?0
Output:?[0]
Explanation: We define the gray code sequence to begin with 0.
?            A gray code sequence of n has size = 2n,which for n = 0 the size is 20 = 1.
?            Therefore,for n = 0 the gray code sequence is [0].

代码实现：

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